1. Field of the Invention
This invention relates to integrated circuit (IC) packages, and more particularly, to a method of forming a constricted-mouth dimple structure on a die pad of a leadframe, which can help secure the molded compound of the integrated circuit package more securely to the die pad.
2. Description of Related Art
The integrated circuit packaging technology is used to pack an integrated circuit chip, which is typically very small in size, in a molded compound so that it can be easily mounted onto a printed circuit board. An integrated circuit package typically utilizes a leadframe having a die pad for mounting the integrated circuit chip, and a molded compound is formed to encapsulate the integrated circuit chip and the die pad.
FIG. 1 is a schematic sectional diagram showing the inside structure of a typical integrated circuit package, which includes a leadframe die pad 10, an integrated circuit chip 20 mounted on the die pad 10, and a molded compound 30 encapsulating the die pad 10 and the integrated circuit chip 20. To allow the molded compound 30 to be more firmly secured to the die pad 10, it is a common practice to form a plurality of dimples 11 on the back side of the die pad 10 for the purpose of increasing the contact area between the molded compound 30 and the die pad 10. This can help increase the bonding strength between the molded compound 30 and the die pad 10, making the molded compound 30 more firmly secured in position to the die pad 10.
Conventionally, these dimples 11 can be formed either by an etching process or a stamping process. These two processes would form the dimples into different shapes, as respectively depicted in the following with reference to FIGS. 2A-2B.
FIG. 2A is a schematic sectional diagram of a leadframe die pad 10 on which a plurality of dimples 11 are formed through an etching process. As shown, due to the nature of the etching process, these dimples 11 would be formed into a substantially semi-circular shape in cross section. FIG. 2B is a schematic sectional diagram of a leadframe die pad 10 on which a plurality of dimples 12 are formed through a stamping process. As shown, due to the nature of the stamping process, these dimples 12 would be formed into a substantially trapezoidal shape in cross section.
One common quality of the above-mentioned two types of dimples 11, 12 is that the width thereof becomes smaller from the mouth toward the inside, i.e., the mouth is the widest while the bottom is the narrowest in dimension (this kind of dimple structure is herein and hereinafter referred to as "inwardly-tapered dimple structure" throughout this specification).
One drawback to the inwardly-tapered dimple structure is that it would be unable to act as a anchor to the molded compound. In other words, when the molded compound is forcibly pulled, it would easily cause the molded compound to be drawn off the inwardly-tapered dimple structure, making the molded compound easily subject to delamination.
One solution to the foregoing problem is to form a plurality of slots in the die pad so as to catch the molded compound securely in position. One drawback to this solution, however, is that it would make the die-attachment process become more complex and thus difficult to carry out.
Another solution to the foregoing problem is disclosed in the U.S. Pat. No. 5,367,191 "LEADFRAME AND RESIN-SEALED SEMICONDUCTOR DEVICE". This patent utilizes two materials of different etching rates to form the die pad so that after an etching process is performed on the die pad, it can allow the resulted dimples to be formed with a constricted mouth (See FIG. 3 of the U.S. Pat. No. 5,367,191 Specification, for the part designated by the reference numeral 14). This kind of dimple structure is herein and hereinafter referred to as "constricted-mouth dimple structure" throughout this specification.
Due to the fact that a constricted-mouth dimple structure has a wide inside space and a constricted mouth, it can act as a anchor to the infilled part of the molded compound, thus preventing the molded compound from being pulled off the die pad, making the molded compound more firmly secured in position to the die pad.
One drawback to the forgoing patent, however, is that the use of two different materials of different etching rates to form the die pad would undesirably increase the material cost and make the overall manufacture process more complex to carry out. Still one drawback is that since the die pad has two different etching rates, the dimension control on the die pad would be difficult to carry out.